The long-term objective of this proposal is to define the molecular scaffolding that underlies the dynamic architecture of vertebrate rod and cone photoreceptor outer segments (OSs). Retinal photoreceptors provide important paradigms for modern biology, including mechanisms underlying G-protein mediated signal transduction and neurodegenerative diseases. Although excellent progress has been made in describing the phototransduction cascade, the structural organization of the photoreceptor outer segment (OS) remains poorly understood at the molecular level. We propose to advance knowledge of OS architecture to provide a basis for understanding how scaffolding defects impair rod and cone cell viability to cause retinal disease. Our previous studies have elucidated structure/function relationships for peripherin/rds (P/rds), a photoreceptor-specific tetraspanin that plays an essential role for rod and cone cell integrity, and generates a uniquely broad range of progressive retinal diseases when defective. We will evaluate the hypothesis that P/rds is multifunctional, and that its self-assembly, protein scaffolding, and membrane binding activities, represent keystones for its support of OS structure and photoreceptor viability, and involvement in diverse disease phenotypes. Specific Aim 1 will elucidate relationships between known P/rds interactions, OS disk morphogenesis, and retinal disease. Specific Aim 2 will identify and validate novel OS scaffolding interactions. Specific Aim 3 will elucidate the significance and regulation of P/rds cytoplasmic activities. In sum, these studies address critical gaps in current understanding of photoreceptor cell biology, and the molecular basis of pathogenicity caused by RDS defects. They advance our molecular dissection of photoreceptor architecture, and will provide a new system for the discovery and in vivo analysis of protein-protein interactions in vertebrate photoreceptors. Retinal photoreceptors provide the cellular basis for sight. These fragile cells use a highly organized "outer segment" to detect incoming photons. Disorganization of outer segment architecture by trauma or inherited disease can impair photoreceptor function and viability to cause a wide range of retinal diseases. This project will provide a better understanding of photoreceptor outer segment structure in health and disease.